The present invention relates to methods and apparatus for providing access priority control in a communications system and, more particularly, to methods and apparatus for providing access priority control in a media access control protocol of a Universal Mobile Telecommunications System.
A major effort has been underway in the last decade to integrate multimedia capabilities into mobile communications. The International Telecommunications Union (ITU) and other organizations have been attempting to develop standards and recommendations that ensure that mobile communications of the future will be able to support multimedia applications with at least the same quality as existing fixed networks. Particularly, many global research projects have been sponsored in order to develop such next (third) generation mobile systems. Research and Development of Advanced Communication Technologies in Europe, RACE-1, and RACE-2, and Advanced Communications Technology and Services (ACTS) are examples of such efforts in Europe. It is known that in order to provide end users with the requisite service quality for multimedia communications, Internet access, video/picture transfer, high bit rate capabilities are required. Given such requirements, bearer capability targets for a third generation system have been defined as 384 kilobits per second (kb/s) for full coverage area and 2 Megabits per second (Mb/s) for local area coverage.
Universal Mobile Telecommunications System (UMTS) is a new radio access network based on 5 Megahertz Wideband Code Division Multiple Access (W-CDMA) and optimized for support of third generation services including multimedia-capable mobile communications. Since major design goals of UMTS are to provide a broadband multimedia communications system that integrates infrastructure for mobile and fixed communications and to offer, inter alia, the same range of services as provided by the fixed and wireless communications networks, UMTS must provide circuit-switched as well as packet-switched services, a variety of mixed-media traffic types, and bandwidth-on-demand. However, providing multimedia support implies the need for flexibility, that is, being able to support services with different bit rates and Eb/N0 requirements, and to multiplex such services in a multiservice environment. UMTS is designed to be able to support such demands.
Referring to FIG. 1, an exemplary block diagram of a UMTS access network is shown. Particularly, a plurality of remote terminals 2 and 4 (e.g., mobile terminals) communicate with base stations (NODE-B) 6 via W-CDMA wireless links 8. The remote terminals may be a variety of devices such as a wireless phone 2 or a portable personal computer 4 with an internal or external modem. In the UMTS standard, a base station is called a NODE-B. These base stations communicate with a network component that provides radio resource management functions and is called a Radio Network Controller (RNC). Since UMTS is a W-CDMA system, soft handoffs are supported. In the case of soft handoffs, there are two base stations 6 serving one remote terminal. Thus, the remote terminal sends frames to these two base stations. When the two base stations receive the frames from the remote terminal, they send them to a Frame Selector Unit (FSU). The FSU decides which is a better frame, in terms of frame quality, to be sent to the core network. In UMTS, the FSU may be physically integrated with the RNC and as such, in FIG. 1, the RNC and FSU are shown as block 10, but also are separated functionally as block 12 (FSU) and block 14 (RNC). Other elements in the UMTS network perform conventional functions such as: the xLR databases 20, which provide home and visiting location information; and the interworking function (IWF) units 22. It is to be appreciated that the Universal Mobile Switching Center (UMSC) 16 serves as the mobile switching center for the base stations 6 in the UMTS. Sub-networks 18 are wireless service provider networks and CNI through CNn are the core networks 24 to which the remote terminals are ultimately coupled.
Referring to FIG. 2, a diagram of the typical protocol stack in UMTS is shown. In. UMTS, Layer 1 (L1) is the physical layer (PHY) which offers information transfer services to the MAC (Media Access Control) layer and higher layers. The physical layer transport services are described by how and with what characteristics data is transferred over the transport channels of the radio interface. Layer 2 (L2) is comprised of sublayers which include MAC, LAC (Link Access Control), and RLC and RLCxe2x80x2 (Radio Link Control). In UMTS, the functions performed in RLC are split and thus two RLC protocols (RLC and RLCxe2x80x2) are specified. The RLC and MAC layers provide real-time and non-real-time services. The MAC layer controls but does not carry out the multiplexing of data streams originating from different services. That is, the MAC layer, via logical channels, allows common physical communications channels (e.g., broadcast channel) to be shared by a number of remote terminals. IP (Internet Protocol) is the network layer.
xe2x80x9cUuxe2x80x9d refers to the UMTS-specific interface between a remote terminal and a base station, while xe2x80x9clubxe2x80x9d refers to the UMTS-specific interface between a base station and the RNC/FSU. Layer 2 of the radio access network (i.e., left side of NODE-B on the protocol stack) is split into RLC and MAC layers, while Layer 2 of the core network (i.e., right side of NODE-B on the protocol stack) is more related to the technology used to transport network layer frames, e.g., ATM (Asynchronous Transfer Mode) or Frame Relay. IP is shown as the transport protocol, however, UMTS is not so limited. That is, UMTS can cater to other transport protocols. Further details on the protocol layers may be found in Dahlman et al., xe2x80x9cUMTS/IMT-2000 Based on Wideband CDMA,xe2x80x9d IEEE Communications Magazine, pp. 70-80 (September 1998) and in ETSI SMG2JUMTS L2 and L3 Expert Group, xe2x80x9cMS-UTRAN Radio Interface Protocol Architecture; Stage 2,xe2x80x9d Tdoc SMG2 UMTS-L23 172/98 (September 1998).
In UNMTS, four types of application traffic need to be handled. They include: (i) applications that are both delay and loss sensitive, e.g., interactive video; (ii) applications that are loss sensitive but can tolerate moderate delay, e.g., interactive data; (iii) applications that are delay sensitive but tolerant of moderate losses, e.g., voice; and (iv) applications that are tolerant of both delay and losses, e.g., file transfer.
To provide different Quality of Service (QoS) to all these different applications, the UMTS system must be designed appropriately. Several important issues need to be considered in UMTS system design such as, for example, how to satisfy QoS without wasting network resources and how to operate the systems in the stable region when all traffic types burst simultaneously.
Further, several components are required in UMTS to support varying QoS. For example, service parameters need to be defined to enable different applications to specify their different QoS requirements, e.g., the Guaranteed Service and Controlled Load Service parameters defined by the Internet Engineering Task Force (IETF). Users can ask for bandwidth resources either on a burst mode or connection mode. Also, there must be an admission control component in UMTS that makes decisions as to whether or not users"" requests will be granted. The admission of new requests must be done such that even when all admitted requests peak simultaneously, the QoS requirements of each request will not be violated (unless they are best effort requests). Further, once the users"" requests are admitted, there must be features implemented in the UMTS network to deliver such service guarantees, e.g., delay requirement, packet loss requirement. Scheduling algorithms at the network nodes and packet marking for non-conformant users"" traffic are some of the features that can be supported by routers to provide differentiated services.
In order to provide end-to-end QoS in UMTS, certain features need to be provided at the MAC layer to ensure different QoS. One possible way of providing different QoS is by providing priority mechanisms. Priority mechanisms can be implemented in terms of access priority, service priority or buffer management schemes. There are various types of service priority mechanisms, e.g., fixed priority, dynamic priority. Fixed priority mechanisms include, e.g., strict priority and weighted round robin. Dynamic priority schemes include, e.g., fair share queuing, self-clock fair share queuing and worst case fair share queuing disciplines.
With respect to access priority, several well-known channel access protocols are currently used in wireless data systems, such as Slotted Aloha, PRMA, etc. Conventional Slotted Aloha is a relatively simple protocol but, because it does not attempt to avoid or resolve collisions between data users, its theoretical capacity is just 0.37.
Reservation-based protocols attempt to avoid and resolve collisions by dynamically reserving channel bandwidth for users needing to send packets. Typically, in such protocols a channel is divided into slots that are grouped into frames of N slots. A slot can be further subdivided into k minislots. Normally, A1 of the slots will be used for reservation purposes while the remaining A-A1 slots are data slots. The users that need to send packets send a reservation request packet in one of the B=A1*k minislots. If the reservation request packet is successful, then the user will be allocated a certain number of data slots until the user or the base station releases the reservation. If the reservation request packet is not successful, the user will use a conflict resolution method to retransmit the reservation request until it is successfully transmitted.
Access priority control is particularly critical with respect to one of the logical channels associated with the media access control (MAC) protocol of UTMS, namely, the random access channel (RACH). RACH is an up-link common transport channel used to carry control information and short user packets from a remote terminal. Referring to FIG. 3, a block diagram of an exemplary hardware implementation of a non-coherent RACH detection algorithm for use in a UMTS base station (NODE-B in FIG. 1) is shown. The RACH receiver 30 is capable of providing the following functions: detection, demodulation and decoding, and acknowledgement. The purpose of detection is to determine if a RACH burst, described below, is being sent by a remote terminal and to resolve the strongest multipath components of the incoming burst. The receiver 30 also demodulates and decodes the message contained within the corresponding RACH to ascertain the remote terminal identifier and the requested service. After decoding a remote terminal RACH transmission, the receiver generates an acknowledgement signal which the base station transmits to the remote terminal over a Forward Access Channel (FACH).
The RACH receiver 30 preferably performs the above functions in accordance with the following structure. A RACH transmission burst is received and demodulated by mixers 32 and then filtered in filters 34. The signal is then sampled in sampling unit 36. Despreader 38 decodes the signal in accordance with the spreading sequence, in this case, 512 Gold code. The decoded signal is buffered (buffer 40) and sent to time shifting unit 50. Also, the output of the despreader 38 is provided to integrator 42. The outputs of the integrator 42 are mixed (mixer 44) and provided to timing detector 46 and then threshold detector 48. The output of the threshold detector 48 indicates whether a valid signal was received from the remote terminal. This result is provided to time shifting unit 50. If it is a valid signal (e.g., above predetermined thresholds), the decoded signal is then down-sampled by unit 52. Then, depending on the preamble, described below, the signal passes through the 16 tap filter unit 54 to the preamble signature searcher 56. The output of the searcher 56 provides the base station with the remote terminal""s identifier and information as to the service(s) requested by the remote terminal.
It is known that the physical RACH is designed based on a Slotted ALOHA approach. A remote terminal can transmit a random access burst 100 at eight well-defined time offsets (Access slot #1, . . . , Access slot #i, . . . , Access slot #8) relative to the frame boundary of the received broadcast control channel (BCCH) of the current cell, as illustrated in FIG. 4A. As shown in FIG. 4B, the random access burst consists of two parts, a preamble part 102 of length 1 millisecond (ms), a message part 104 of length 10 ms, and an idle time 106 of length 0.25 ms in between the preamble part and the message part. There are a total of 16 different preamble signatures that are based on the Orthogonal Gold code set of length 16 (512 Gold code). The information on the available signatures and time offsets are broadcast on BCCH. Based on this structure, if the receiver has 128 (16 preamble signatures multiplied by 8 timeslots) parallel processing units, 128 random access attempts can be simultaneously detected. In other words, we have equivalent 128 random access channels for a maximum configured base station for the current cell.
Accordingly, there is a need for methods and apparatus for providing access priority in UMTS that addresses the unique requirements associated with such a broadband multimedia communications system. Specifically, there is a need for methods and apparatus for providing access priority with respect to UMTS RACH.
The present invention provides methods and apparatus for providing access priority in a MAC protocol of a communications system such as, for example, with respect to UMTS RACH. Particularly, the invention introduces several access priority methodologies including: (i) random chip delay access priority (RCDAP); (ii) random backoff based access priority (RBBAP); (iii) variable logical channel based access priority (VLCAP); (iv) a UMTS-specific variation to the variable logical channel based access priority scheme (VLCAPxe2x80x2); (v) probability based access priority (PBAP); and (vi) retransmission based access priority (REBAP).
In one aspect of the invention, RCDAP methods and apparatus are provided. In RCDAP, each priority class is advantageously assigned a different chip delay from among chip delay distributions prior to submitting an access request to the base station. Preferably, those classes with a higher priority are given a smaller average random chip delay such that their access requests will have a higher probability of being captured compared to those submitted by users with a lower priority class.
In another aspect of the invention, RBBAP methods and apparatus are provided. In RBBAP, each priority class is advantageously assigned a different backoff delay. Preferably, requests associated with higher access priority will have a smaller average backoff delay. Whenever there is a collision or some other reason an access request is not successfully received at the base station, the remote terminal, depending on the class i, chooses a random delay distributed between a pre-determined range.
In yet another aspect of the invention, VLCAP methods and apparatus are provided. In VLCAP, each subscriber is given an access priority class i. Preferably, those with the highest priority can access all the logical access channel for which the base station is configured, while those with lowest priority are only allowed to access a small subset of logical access channels, e.g., only one preamble signature with 8 time offsets. A rationale behind this approach is that the larger the number of logical access channels that the remote terminal has to choose from, the higher the likelihood of finding a channel on which the request will be successfully transmitted.
In a further aspect of the present invention, a UMTS-specific variation of VLCAP methods and apparatus are provided. The VLCAPxe2x80x2 approach, specifically takes into account a special UMTS access channel structure. That is, even though there are t time offsets for each preamble signature, there may not be t parallel processing units at the base station due to a limitation on the processing complexity associated with the base station. For example, there may only be four receivers with each receiver programmed to capture, for example, the (ith, (i+4)th) time offsets. Thus, according to the VLCAPxe2x80x2 approach, those requests with lower priority classes will be assigned a higher number for the time offsets, thus allowing the access requests from higher priority classes to be captured by the receivers first.
In still a further aspect of the invention, PBAP methods and apparatus are provided. In PBAP, each subscriber is given an access priority class i. Each access priority class i can only transmit access requests with a certain probability Pi. Those with the highest priority always transmit their access requests whenever they have an access request.
In yet another aspect of the invention, REBAP methods and apparatus are provided. In REBAP, access requests have an access packet priority (APP) associated therewith whereby retransmitted access requests are given a higher priority over new access requests.
It is to be appreciated that access priority techniques implemented according to the present invention may include a combination of more than one of the above embodiments. For example, RCDAP can be performed with RBBAP or VLCAP and PBAP, and so on.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.